Noble metals having a high emittance coating of iron titanate

ABSTRACT

A method of applying a high-emittance coating to the noble metals and intra-alloys thereof, the coating being applied by plasma arc spraying iron titanate powder on the surface of the noble metal. A substrate of a noble metal or intra-alloy thereof having a high emittance coating of iron titanate bonded thereon.

United States Patent 1191 Carroll [45 1 Aug. 21, 1973 NOBLE METALSHAVING A HIGH EMITTANCE COATING OF IRON TITANATE [75] Inventor: David F.Carroll, Hermosa Beach,

Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif.

22 Filed: Dec. 4, 1967 [21] Appl. No.: 687,594

[52] US. Cl 29/195, 29/194, 29/198,

117/217,117/93.1, l17/221,165/133,161/225 [51] Int. Cl C23c 5/00, C23d5/10 [58] Field of Search 117/93.l,201,2l7,

[56] References Cited UNITED STATES PATENTS 3,183,337 5/1965 Winzeler etal 117/93.1 X

3,246,114 4/1966 Matuay 117/93.1 X 3,347,698 10/1967 lngham 117/911 XFOREIGN PATENTS 0R APPLICATIONS 1,003,118 9/1965 Great Britain 1l7/93.1

Primary Examiner-Alfred L. Leavitt Assistant Examiner-J. H. NewsomeAttorney-Ro1and A. Anderson [57] ABSTRACT A method of applying ahigh-emittance coating to the noble metals and intra-alloys thereof, thecoating being applied by plasma arc spraying iron titanate powder on thesurface of the noble metal.

A substrate of a noble metal or intra-alloy thereof having a highemittance coating of iron titanate bonded thereon.

5 Claims, 3 Drawing Figures Patented Aug. 21, 1973 Dovi E CarrollINVENTOR.

ATTORNEY NOBLE METALS HAVING A HIGH EMITTANCE COATING OF IRON TITANATEBACKGROUND OF THE INVENTION Controlling the surface emittance ofradioisotope containment vessels for aerospace applications has been adifficult problem in the prior art. This problem has been particularlysevere in view of the safety requirements which are that no release ofthe radioactive content be allowed under any conceivable situation, thatis, both normal operation and all abnormal modes of operation, includingabort modes. The most probable abort situation involved is exposure of afuel vessel to terrestial gases, such as air or water vapor and carbondioxide, at elevated temperatures up to 2,500 F for an extended periodof time, measured in years. The survivability of the vessel in thissituation depends strongly upon the temperature of the surface of thevessel, and which can be lowered to an acceptable level by the use of ahigh emittance coating according to the invention.

It has been detennined that containment material for radioisotopevessels should include a refractory metal strength member clad with anoble metal. The refractory metals usually include vanadium, niobium(columbium), tantalum, chromium, molybdenum, tungsten and rhenium. Arefractory metal provides high temperature (1,600 E) creep strength andthe noble metal provides protection from the rapid, high temperatureoxidation that a bare refractory metal container would incur. A barenoble metal cladding has a disadvantage and that is its lowhemispherical emissivity of about 0.2.

Hemispherical total emissivity, E is the ratio of the radiancy (rate ofemission of radiant energy from a unit area of a surface) of a polishedsurface to that of a black body at the same temperature.

The steady state temperature of a radioisotope capsule in a ground abortsituation, for example, is determined principally by the emissivity ofits surface: the higher the emissivity, the lower the capsuletemperature. When noble metal claddings are used for oxidationprotection of refractory metals, they degrade at a rate dependentstrongly upon temperature. Thus, any reduction in capsule tempertureachieved by increasing the emissivity of a noble metal cladding willenhance its ground abort survivability.

SUMMARY OF THE INVENTION The invention relates generally to a means toincrease the total emittance of noble metals. It has been surprisinglyfound that the application of a plasma arc sprayed coating of irontitanate on the surface of a noble metal increases the hemisphericalemittance approximately 300 percent. The invention is comprised of amethod to plasma arc spray the noble metals or intranoble alloys thereofwith a coating of iron titanate powder.

Accordingly, an object of the invention is to provide a method forcoating noble metals and intra-alloys thereof to produce a noble metaland coating having a high emittance.

Another object of the invention is to provide a high emittance claddingformed of the noble metals and intra-alloys thereof.

Still another object of the invention is to provide a high emittancecoating for noble metals that is stable in air at up to 2,500 F. Afurther object of the invention is to provide a high emittance coating,as described in the previous paragraphs, which is ductile.

Still further object of-the invention is to provide a coating, asdescribed in the previous paragraphs, and with which no deleteriouschemical interactions occur between the coating and the noble metals.

Further objects and advantages of the invention may be brought out inthe following part of the specification wherein small details have beendescribed for the competence of disclosure, without intending to limitthe scope of the invention which is set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the accompanying;drawings, which are for illustrative purposes:

FIG. I is an isometric view of a :noble metal substrate having a highemittance coating thereon in accordance with the present invention;

FIG. 2 is a magnified end view of a substrate of a platinum-rhodiumalloy having an iron titanate coating bonded on opposite'sides thereofin accordance with the invention; and

FIG. 3 is an illustration of a cladding for a refractory metal andformed of a noble metal tube with inner and outer surfaces coatedwithiron titanate in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventive process iscomprised of plasma arc spraying iron titanate powder, having a particlesize range of 44 to 74 microns, on a surface of one of the metalsselected from the group containing ruthenium, rhodium, palladium,osmium, iridium and platinum and intralloys thereof. Either the naturaliron titanate, the mineral ilmenite or synthetic iron titanate,FCgOa'TIOz, can be used. The thickness of the iron titanate coating mayvary from approximately 1.0 mil to several mils. If required, thesurface of the coating can be ground to specified dimensions. When thecoated noble metal is used as a cladding for a refractory metal, such asvanadium, niobium (columbium), tantalum, chromium, molybdenum, tungstenand rhenium, it is generally formed as a thin substrate sheet having athickness of between 0.020 and 0.050 inch.

Prior to coating a substrate of one of the noble metals, it may bedesirable to anneal metal to relieve any mechanical stresses. Thesurface of the noble metal is then cleaned to remove organiccontaminants, prefera bly with ethyl alcohol because it leaves no film.Next, the surface of the metal to be coated is activated by, forexample, grit blasting with glass beads or alumina. The activationincreases the adherence of the iron titante to the surface. After thesurface has been activated, it is again cleaned with ethyl alcohol,preferably in an ultrasonic cleaner to shake out most of the particlesembedded during the grit blasting. The substrate is then ready forplasma arc spraying, preferably using an argon gas plasma.

The process described above unexpectedly resulted in the production of aductile, stable, high emittance coating for nobel metals. A' strip ofplatinum-l0 weight percent rhodium, /4 inch wide, 0.1 inch thick and 8inch long, coated with plasma arcsprayed iron titanate, can be bentaround a 2 inch radius with no cracking.

is made with the uncoated platinum-l weight percent rhodium alloy:

Total hemispherical emittance Material 1,600 F. 1,800 F. 2,000" F.2,200" F. 2,400 F.

Platinumweight percent rhodium (uncoated) 0. 16 0.16 0. 17 0.17 0. 18Plasma are sprayed iron titanate on platinum-10 weight percent rhodium0.86 0. 85 0. 84 0. 84 0. 86 Plasma are sprayed iron titanate onplatinum-10 weight percent rhodium exposed for 500 hours to flowing airat ,000 F 0. 82 0. 82 0. 82 0. 82 Plasma are sprayed iron titanate onplatinum-i0 weight percent rhodium exposed for 980 hours to flowing airat 2,000 F 0. 86 0. 85 0.85 0.85 O. 86

plate 10 having bonded (mechanical-chemical bond) on one side 11 with athin layer of plasma arc sprayed iron titanate 12. The bonding is shownby the irregular surfaces as at 13 where the iron titanate is joined tothe noble metal. This can generally be seen only through highmagnifications.

in FIG. 2, there is shown a substrate 17 with platinum-l0 weight percentrhodium alloy, having bonded on two opposite sides a layer of irontitanate 18 which was plasma arc sprayed onto the substrate. The drawingin FIG. 2 was made in exact size from a copy of a 200K photomicrograph.

The platinum-rhodium alloy has its normal whitish color, and the irontitanate is a medium gray having black spots 19 which are shadows formedin cavities. There are rough surface bonding lines 20 and 21 between theiron titanate and the platinum-rhodium alloys, illustrating theadherence of the iron titanate coating to the noble metal. After heatingthe substrate shown for 980 hours in air at 2,000 F, no chemicalinteractions occurred between the coating and the noble metal.Microhardness measurements across the noble metal substrate alsoindicated no changes in the metal because of the iron titanate coating.The thickness of the two coating layers and the substrate, as may beused for cladding on radioisotope containment vessels, is in the rangeof 22 mils and 70 mils, and the thickness of the individual irontitanate coating is in a range of from 1 mil to 10 mils.

in FIG. 3, there is shown a tube 24 having an inner and outercylindrical coating 25 and 26, respectively, of iron titanate on a tubeportion 27 of a noble metal in the relationship as shown in FIG. 2. Thethicknesses of the coating and the noble metal are exaggerated and areshown to illustrate a tubular cladding for use on a refractory metal forcontaining radioisotopes.

The following examples illustrate a way of carrying out the invention asotherwise described above:

1. The noble metal or intra-alloy of a noble metal is cleaned with ethylalcohol to remove to organic contaminants.

2. The surface of the noble metal to be coated is grit blasted withglass beads to activate its surface to increase the adherence of thecoating to the surface.

3. The grit-blasted surface is cleaned with ethyl alcohol, preferably inan ultra-sonic cleaner to shake the particles off the surface.

4. The surface is plasma arc sprayed, using argon gas plasma, with 200mesh, +325 mesh iron titanate powder, which freezes on the surface.

The measured emittance values and stability of the iron titanate coatingon the noble metals can be seen from the data in the table below, inwhich a comparison As can be seen from the table, the total emittance ofplatinum-l0 weight percent rhodium is increased by the iron titanatecoating approximately 300 precent from 0.2 to 0.8. The stability in airat 2,000 F is shown to be good, that is, exposure to air at hightemperatures does not appreciably change the emittance values.

The invention and its attendant advantages will be undetstood from theforegoing description, and it will be apparent that various changes maybe made in the form, construction and arrangement of the parts of theinvention without departing from the spirit and scope thereof orsacrificing its material advantages, the arrangement hereinbeforedescribed as being merely by way of example. I do not wish to berestricted to the specific forms shown or uses mentioned, except asdefined by the accompanying claims, wherein various portions have beenseparated for clarity of reading and not for emphasis.

I claim:

1. A substrate of a platinum-group metal having a high emittancecoating, comprising:

a. said metal being one selected from the group consisting of ruthenium,rhodium, palladium, osmium, iridium and platinum and intra-alloysthereof; and

b. an iron-titanate coating on at least one surface of said metal.

2. The invention according to claim 1 in which: said iron titanatecoating has been plasma arc sprayed on said metal.

3. The invention according to claim 1 in which: said coating has athickness of at least approximately 1.0 mil.

4. The invention according to claim 1 in which:

said coated metal has a hemispherical emittance range of approximately0.82 to 0.86 in the temperature range of from 1,600 F to 2,400 F.

5. A refractory metal having a high emittance cladding comprising:

A. a refractory metal selected from the group consisting of vanadium,niobium, tantalum, chromium, molybdenum, tungsten, and rhenium on whichis clad B. a noble metal selected from the group consisting ofruthenium, rhodium,palladium, osmium, iridium, and platinum andintraalloys thereof, said noble metal having a coating of C. irontitanate.

UNITED STATES PATENT OFFICE CERTIFICATE 0F QORRECTION Patent No.3,753,666 Dated August 21, 1973 Invent David F. cal-r011 It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

after [75] add --[73] Assignee: The United States of America asrepresented by the United States Atomic Energy Commission, Washington,

Do Ct Signed and sealed this 23rd day of April 19714.

(SEAL) At he s t EDWARD I-I.FLETC1:IER JR. C MARSHALL DANN AttestingOfficer C mmissioner of Patents FORM P0-1050 (10-69) USCQMM-DC 60376-P69fi' LLS. GOVERNMENT PRINTING OFFICE: "09 0-35-334.

2. The invention according to claim 1 in which: said iron titanatecoating has been plasma arc sprayed on said metal.
 3. The inventionaccording to claim 1 in which: said coating has a thickness of at leastapproximately 1.0 mil.
 4. The invention according to claim 1 in which:said coated metal has a hemispherical emittance range of approximately0.82 to 0.86 in the temperature range of from 1, 600* F to 2,400* F. 5.A refractory metal having a high emittance cladding comprising: A. arefractory metal selected from the group consisting of vanadium,niobium, tantalum, chromium, molybdenum, tungsten, and rhenium on whichis clad B. a noble metal selected from the group consisting ofruthenium, rhodium,palladium, osmium, iridium, and platinum andintraalloys thereof, said noble metal having a coating of C. irontitanate.